In a circuit configuration of a conventional battery pack system illustrated in FIG. 1, a battery charger 3e had controlled a charging voltage and a charging current so as to supply the charging voltage and the charging current to a battery pack 1e. For example, as illustrated in FIG. 2, a battery charge had been controlled by a method of a CCCV charge, i.e. recharging at a set current Is in a constant current charge period T1, and recharging at a set voltage Vs in a constant voltage charge period T2.
Thus, the conventional battery pack system had recharged a secondary battery (hereinafter, referred to as a battery) in the battery pack, while the battery charger had monitored the charging voltage and the charging current.
Meanwhile, there were cases where the conventional battery pack system could not recharge the battery in the battery pack appropriately depending on conditions of a temperature of the battery pack and an internal resistance of the battery.
As for a technique for the conventional battery pack system, a battery charger described in PTL 1 has been known. The battery charger includes a first charge controller for recharging a battery so that a charge amount of the battery becomes a first charge amount, a second charge controller for recharging the battery so that the charge amount of the battery becomes a second charge amount smaller than the first charge amount, and a selector for selecting between charge processing by the first charge controller and charge processing by the second charge controller.
In other words, the conventional battery charger selects between a charge method for prioritizing battery life and a charge method for prioritizing a reduction of a charge time when recharging the battery. The battery charger can also select other methods depending on temperature and user purposes.